Thermoplastic resins are variously utilized as molding materials for household appliance parts, electronic and electric parts, OA device parts, audio and imaging device parts and automobile parts.
Many of such thermoplastic resins accumulate static electricity because they are electric insulating materials. The accumulation of static electricity leads to dust adhesion and electrostatic discharge. The accumulation further causes very serious trouble such as breakage of ICs, transistors, circuit substrates and the like, which are vulnerable to static electricity.
Therefore, many modifications and ingenuities have been proposed such as imparting antistaticity to thermoplastic resin compositions having electric insulation by formulating them with conductive substances.
Conductivity-imparted noninsulating resin compositions have largely different performances depending on electric resistivities thereof. The compositions are generally classified as follows by range of the surface resistivity.
(1) A conductive resin composition, which has a surface resistivity of less than 1×105 Ω/sq., and which causes severe static discharge in contact with a charged object, and exhibits a high conductivity (a low resistivity).
(2) A static dissipative resin composition, which has a surface resistivity of from 1×105 to 1×109 Ω/sq., which does not cause severe static discharge in contact with a charged object, and exhibits a conductivity dissipating the charge promptly, and which does not have a conductivity enough to shield the static field.(3) An antistatic resin composition, which has a surface resistivity of from 1×109 to 1×1014 Ω/sq., and which has a conductivity capable of preventing the charging of itself to some degree, but does not have a conductivity enough to dissipate promptly static electricity of a charged object.
Documents describe techniques to impart conductivity to various types of thermoplastic resins by formulating the resins with various types of conductive materials. For example, proposed are a resin composition in which conductive carbon black, natural scaly graphite and an inorganic filler are formulated in a polyphenylene sulfide (for example, see Patent Document 1), a resin composition in which conductive carbon black, graphite and a filler are formulated in a polyphenylene sulfide resin (for example, see Patent Document 2), a resin composition in which carbon fiber, graphite, a silane-based coupling agent and an epoxy resin are formulated in polyarylene sulfide, (for example, see Patent Document 3), a resin composition in which zinc oxide whisker and the like are formulated in a thermoplastic resin (for example, see Patent Document 4), a resin composition in which conductive carbon black and artificial graphite are formulated in a thermoplastic resin (for example, Patent Document 5), a resin composition in which conductive carbon black, graphite and an epoxy group-containing α-olefinic copolymer are formulated in a polyarylene sulfide (for example, see Patent Document 6), a resin composition in which graphite is formulated in a liquid crystal polyester (for example, see Patent Document 7), a resin composition of a semiconductive film in which a conductive filler is formulated in a polyphenylene sulfide (for example, see Patent Document 8), and a resin for a coil encapsulating material (see Patent Document 9).
The noninsulating resin compositions having three electric characteristics described above are suitably selectively used from the range of the surface resistivity according to purposes and applications. Therefore, control of the electric resistivity is important in the technique to impart conductivity to resin compositions.
Especially the static dissipative resin composition and the antistatic resin composition have a largely different surface resistance value and volume resistance value of molded products. The reason is because the surface resistance value becomes less by receiving an influence of leak current to the thickness direction of molded products. However, for plastic members, noninsulating resin compositions are desired which have a stable actually measured surface resistance value and volume resistance value, both of which are nearly equivalent to each other. For obtaining such resin compositions, the techniques described above do not work enough.
Further, remolded products obtained by reutilizing molded products, runner sections, spool sections and the like at molding have a largely varied surface resistivities and a largely different surface resistance value and volume resistance value. For improving this point, the techniques described above do not work enough.    Patent Document 1: Japanese Patent Application No. 62-172059    Patent Document 2: Japanese Patent Laid-Open No. 1-272665    Patent Document 3: Japanese Patent Laid-Open No. 1-254766    Patent Document 4: Japanese Patent Laid-Open No. 5-247351    Patent Document 5: Japanese Patent Laid-Open No. 7-286103    Patent Document 6: Japanese Patent Laid-Open No. 10-158511    Patent Document 7: Japanese Patent Laid-Open No. 2000-281885    Patent Document 8: Japanese Patent Laid-Open No. 2006-69046    Patent Document 9: Japanese Patent Laid-Open No. 2006-291076